This urban-regional line (S-Bahn) is a relatively simple addition to the proposed Budapest urban-regional metro. It would use the existing alignment of the ring line east from Kelenföld Station, with separate tracks as necessary. The line (shown light blue) crosses the Danube, passes through Ferencváros Station, and then turns south-east to Kőbánya-Kispest Station, joining the proposed east-west urban-regional line coming from Keleti Station. Ferihegy airport is a logical end point of the line. It would give good interchange, at a new station, with metro line 3 at Ecseri út (shown in green). Other possible locations for new stations are at Bartók Béla út, at the ELTE university campus, and at Száva utca.
The south tangent can be combined with a semi-circular ring S-Bahn line, using the rest of the ring lines. It could terminate at for instance Rákosrendező station, on the main line north from Nyugati Station. However, only the south tangent line has good interchange with the lines to the east and south-east. It could also connect at Kőbánya-Kispest Station with an outer ring line.